Balanced armature electro acoustic transducers have long been fundamental components of communications equipment ranging from telephones to hearing aids. Very early telephones utilized balanced armature transducers in their earpieces and such speakers took on the name of the entire hand piece and became known as “receivers.” In keeping with this commonly used terminology, the terms “speaker” and “receiver” will be used interchangeably in this specification.
In hearing aid applications, balanced armature devices have been used for both microphones and “receivers.” While other technologies, notably “electrets,” have largely supplanted the use of balanced armature transducers as microphones in the specific context of hearing aids, balanced armature devices continue to be the most commonly used technology for “receivers” in present day hearing aids. Most advantageously, balanced armature devices can produce extremely loud sounds with very little power and within a very small geometric volume and footprint.
A limitation to the performance of conventional balanced armature electro acoustic devices, whether used as speakers or microphones, is that their characteristic frequency spectra deviate from being perfectly flat, spectral flatness being one representation of a lack of distortion, a very desirable characteristic for acoustic (and most other) transducers. This spectral deviation or “signature” arises from the fundamental structural properties that are characteristic of all conventional balanced armature devices: the mass and springiness of: the armature itself, the sound producing diaphragm and its chamber(s), and of the connector element and its attachments that link the armature and the diaphragm. More particularly, the beam and connecting rod of the armature, the diaphragm, and even the air and ports into which the air exits all have associated masses and springiness, and the system has a characteristic resonance that reflects the energy exchange between such masses and springs. Numerous techniques have been developed to minimize the disadvantages of this inherent signature, including, for example, the use of so-called “ferro-fluids” for damping the system and improving the transducer's dynamic performance.
Notwithstanding the substantial enhancements to these general types of transducers, room remains for improving and simplifying the frequency signature and minimizing the frictional and other mechanical losses. Substantial room further exists for enhancing the relationship to the non-linear magnetic forces with a corresponding non-linear springiness of the armature/diaphragm. In many applications, it also is desirable to further reduce the size of the transducer. For example, when used in a hearing aid or earphone application, it is desirable to have a transducer that is small enough to comfortably fit within a human auditory canal. Similarly, when used as a component of a device, such as a cell phone, the small size of the transducer allows the size of the device to be minimized.